1. Field of the Invention
This invention generally relates to integrated circuit UC) fabrication and, more particularly, to a method for the in-situ formation of transmission lines from conventional wire connections.
2. Description of the Related Art
IC signals, processor rates, and communication interface bandwidths all continue to increase in speed. High speed signals are best controlled when conducted through connectors having known transmission characteristics, into loads having matched impedances. It is well known to connect circuit interfaces with matching impedances using a coaxial type transmission line (coax). A coax is comprised of a center conductor with a known diameter, a surrounding insulator, with a known dielectric constant and outer diameter (thickness), and a shield. The signal is carried via the center conductor and the shield is connected to a reference voltage, such as ground.
It is also known to fabricate transmission lines internal to, or on top of circuit boards using microstrip, stripline, or waveguide techniques. Schematically, a transmission line, whatever the type, can be thought of as a series inductor and resistor, with a shunt capacitor.
IC devices are formed from a die of an active semiconductor device. The die can be mounted in a hybrid circuit, printed circuit board (PCB), or a package. For environmental protection, the die may be covered by a passivation layer. However, a package is more typically used since it also dissipates heat and provides a lead system for electrical connections. There are many different types of packages including through-hole, surface mount device (SMD) dual/quad, and SMD area array packages. While the package provides protection, it makes access to the die difficult.
FIGS. 1A and 1B are perspective views of a dual in-line package (DIP) and an IC die without a package, respectively (prior art). It is common for a package body or lead frame 100 to have a die attach area 102. The die 106 has electrical contact pads on its top surface. Inner leads 108 connect pads on the die top surface to outer leads or lead frames 110. Once the inner leads are bonded to the lead frames, the package is sealed with ceramic, in a metal can, or in a polyimide. Epoxy resins are also a common choice.
DIP packages are relatively easy to manufacture and easy to integrate into circuit boards. While transmission lines can be fabricated in the IC die, and in the circuit board to which the IC is eventually integrated, the wire bonds formed in the IC, from the die to the chip carrier lead frames are simple wires that do not have a controlled impedance, and which promote cross-talk. Thus, it is becoming increasing impractical to use DIP and other IC packages that rely upon wire bonding, to carry high-speed signals.
One solution to the problem is to minimize the influence of the chip carrier. The IC device shown in FIG. 1B surface mounts to a PCB socket. The input and output contacts of an IC chip are generally disposed in grid-like patterns that substantially cover a surface of the device or in elongated rows which may extend parallel to and adjacent each edge of the device's front surface, or in the center of the front surface.
For example, a surface mounted chip carrier has terminals formed as plated or etched metallic structures on the dielectric. These terminals typically are connected to the contacts of the chip itself by features such as thin traces extending along the chip carrier itself and by fine leads or wires extending between the contacts of the chip and the terminals or traces. These wires are much shorter than the wires bonds used in DIP packages. In a surface mounting operation, the package is placed onto a circuit board so that each terminal on the package is aligned with a corresponding contact pad on the circuit board. Solder or other bonding material is provided between the terminals and the contact pads. The package can be permanently bonded in place by heating the assembly so as to melt or “reflow” the solder or otherwise activate the bonding material.
An IC that is designed to be surface mounted has pads, or flat conductive discs, on its packaging. Many packages include solder masses in the form of solder balls, typically about 1.0 mm and about 0.8 mm (40 mils and 30 mils) in pitch, and 0.4 to 0.5 mm in diameter, attached to the terminals of the package. A package having an array of solder balls projecting from its bottom surface is commonly referred to as a ball grid array or “BGA” package. For a BGA device there are solder bumps on the pads for connection with a BGA socket. The solder bumps typically fit into grips on a BGA socket for connection to a PCB. Other packages, referred to as land grid array or “LGA” packages are secured to the substrate by thin layers or lands (pads) formed from solder. The pads of an LGA require a socket containing springs or some other type of conductive trace to connect the device to the PCB.
One problem with BGA contacts is that the soldering operation is relatively complicated, requiring complicated board processing equipment. Generally, BGA contact ICs are cost effective for ICs having more than 100 pads. However, while wire bonding is more cost effective for smaller pin count packages, the relatively long lengths of the wire bonds are not shielded, and do not have a controlled impedance, which are required in many high frequency applications.
It would be advantageous if IC dies could be connected to chip carriers with transmission lines instead of wire bonds. It would be advantageous if the above-mentioned ICs could be initially fabricated with conventional wires bonds that can be converted in-situ to transmission lines.